1. Field of the Invention
The invention relates to a flexible printed circuit board; in particular, to a flexible printed circuit board that can maintain both its waterproof properties and sensitivity between its switches.
2. Description of the Related Art
A known printed circuit board is an indispensable component in electric and/or electronic devices. In a printed circuit board in general, a circuit, comprising a plurality of lines, for electrically connecting IC-chips, resistors, capacitors and so forth, is made by conductive thin copper foil lines and an insulating base plate, on which the circuit is printed. A printed circuit board having such a structure has been widely used due to its accuracy for circuit arrangement, compactness, and easy assembly for installation into devices.
As a type of improvement from the printed circuit board, a flexible printed circuit board (FPC) has been developed. In addition to the merits mentioned for the printed circuit board, a flexible printed circuit board can be laid out with a high degree of freedom due to flexibility in narrow spaces in machines and devices. This has been considered a merit of flexible printed circuit boards. Another is its waterproof properties.
The conventional flexible printed circuit board is commonly divided into two types: two-layer and three-layer.
A two-layer flexible printed circuit board 10 is shown in FIG. 1a, and comprises a first circuit layer 11, a second circuit layer 12, a first adhesive layer 13 and a second adhesive layer 14.
The first circuit layer 11 is provided with a plurality of first through holes 111, a plurality of first switches 112, and a first circuit 113 for electrically connecting the first switches 112. The second circuit layer 12 is provided with a plurality of second through holes 121 corresponding to the first through holes 111, a plurality of second switches 122 corresponding to the first switches 112, and a second circuit 123 for electrically connecting the second switches 122. The second switches 122, disposed on the second circuit layer 12, face the first switches 112 disposed on the first circuit layer 11. That is, the second switches 122 are disposed on a surface, facing the first circuit layer 11, of the second circuit layer 12, and the first switches 112 are disposed on a surface, facing the second circuit layer 12, of the first circuit layer 11.
The first adhesive layer 13 is disposed on the first circuit layer 11 in a manner such that it surrounds the first through holes 111 and the first switches 112. The second adhesive layer 14 is disposed on the second circuit layer 12 in a manner such that it surrounds the second through holes 121 and the second switches 122.
As shown in FIG. 1b, a cable 15 is electrically connected with the first circuit 113 of the first circuit layer 11 and the second circuit 123 of the second circuit layer 12 respectively.
A predetermined distance must be maintained between the first circuit 113 and the second circuit 123 so as to prevent a short circuit between them. Furthermore, the distance between the first switches 112 and the second switches 122 must be sufficient to keep a predetermined sensitivity between them.
However, in the conventional flexible printed circuit board 10, the distance between the first circuit layer 11 and second circuit layer 12 is maintained only by the adhesive layers so that short circuits between the first circuit 113 and the second circuit 123 occur easily. In addition, the sensitivity between the first switches 112 and the second switches 122 is too great due to the same reason.
A three-layer flexible printed circuit board 20 is shown in FIG. 2a, and comprises a first circuit layer 21, a second circuit layer 22, an intermediate layer 25, a first adhesive layer 23 and a second adhesive layer 24.
The first circuit layer 21 is provided with a plurality of first through holes 211, a plurality of first switches 212, and a first circuit 213 for electrically connecting the first switches 212. The second circuit layer 22 is provided with a plurality of second through holes 221 corresponding to the first through holes 211, a plurality of second switches 222 corresponding to the first switches 212, and a second circuit 223 for electrically connecting the second switches 222. The second switches 222, disposed on the second circuit layer 22, face the first switches 212 disposed on the first circuit layer 21. That is, the second switches 222 are disposed on a surface, facing the first circuit layer 21, of the second circuit layer 22, and the first switches 212 are disposed on a surface, facing the second circuit layer 22, of the first circuit layer 21.
The first adhesive layer 23 is disposed on the first circuit layer 21 in a manner such that it surrounds the first through holes 211 and the first switches 212. The second adhesive layer 24 is disposed on the second circuit layer 22 in a manner such that it surrounds the second through holes 221 and the second switches 222.
The intermediate layer 25, disposed between the first circuit layer 21 and the second circuit layer 22, is provided with a plurality of third through holes 251 and a plurality of fourth through holes 252. Each of the third through holes 251 corresponds to the first through holes 211 and the second through holes 221 respectively, and each of the fourth through holes 252 corresponds to the first switches 212 and the second switches 222 respectively.
As shown in FIG. 2b, a cable 26 is electrically connected with the first circuit 213 of the first circuit layer 21 and the second circuit 223 of the second circuit layer 22 respectively.
As stated above, a predetermined distance must be maintained between the first circuit 213 and the second circuit 223 so as to prevent short circuit between them. Furthermore, the distance between the first switches 212 and the second switches 222 must be sufficient to keep a predetermined sensitivity between them.
In the conventional flexible printed circuit board 20, the distance between the first circuit layer 21 and second circuit layer 22 is maintained by the intermediate layer 25. That is, both the first adhesive layer 23 and the second adhesive layer 24 adhere to the intermediate layer 25.
Since the flexible printed circuit board must be waterproof, the thickness of the first adhesive layer 23 and the second adhesive layer 24 must be enough to maintain the waterproof properties. Hence, because the first adhesive layer 23 and the second adhesive layer 24 cannot directly adhere to each other, the waterproof properties of such flexible printed circuit board 20 is not good. In addition, since both the first adhesive layer 23 and the second adhesive layer 24 are provided with a predetermined thickness, the sensitivity between the first switches 212 and the second switches 222 is insufficient.